This invention relates to a non-irritating composite suture of silk and hydrophobic thermoplastic elastomers containing at least 25% soft segments which composite suture retains the handling qualities of silk and is also capable of retaining at least thirty-two percent of its initial mechanical strength in vivo after eight weeks. This composite suture has surface barrier properties of a monofilament and tissue reaction comparable to common synthetic sutures. This invention also relates to the process for preparing the composite suture.
Many natural and synthetic materials are presently used as surgical sutures. These materials may be used as single filament strands, i.e., monofilament sutures, or as multifilament strands in a braided, twisted or other multifilament construction. Silk does not lend itself to the fabrication of monofilament sutures and is accordingly generally used in one of the multifilament constructions, preferably the braided form. This results in a silk suture having desirable handling characteristics, being sufficiently flexible and having good knot-tying ability and knot security. However, presently available untreated silk sutures are known (a) to provoke a significant tissue reaction in the biologic environment, (b) have a significant strength loss in living tissues (typically a 2-0 silk suture retains about twenty percent of its original strength after eight weeks, post-implantation, and (c) to lack the surface barrier properties needed for retarding cellular infiltration into the suture interior in living tissue.
The composite suture of the present invention displays equivalent handling properties to those of the untreated braided silk suture, it elicits reduced tissue reaction after seven days and later post-implantation intervals, and when implanted intramuscularly, is more effective in retarding cellular infiltration due to the monofilamentous geometry of the composite suture and it is characterized by improved strength retention after fifty-six days post-implantation.
The prior art discloses a number of methods for coating sutures in general. Coating material for sutures normally would require low surface friction characteristics so as to facilitate the knot-tying ability of the resultant coated suture. Contrary to such expectations, the present invention utilizes an elastomer (which has high surface friction characteristics) in preparing the present composite suture.
Braided silk sutures are desirably flexible due to the interlocking geometry of the fibers. in accordance with the present invention, a multifilament silk suture is treated with a hydrophobic, limp thermoplastic elastomer in order, not only to coat the suture, but to substantially fill all the interstices between the silk filaments. It has been found, surprisingly, that the particular elastomers utilized in accordance with the present invention, when filling the spaces between the silk fibers, do not adversely affect the flexibility of the suture as a whole.